Method for forming thermoplastic resin foam

ABSTRACT

The present invention relates to the step of plasticizing a thermoplastic resin material by rotatably driving a screw ( 20 ). Also included in the method is the step of injecting an inert gas into a screw cylinder ( 1 ) to permeate the melted resin and injecting the melted resin into a mold by driving the screw ( 20 ) in the direction of injection. Here, the inert gas has a pressure, at least in pressure, equal to or greater than a supercritical pressure or is under a supercritical state. To provide a thermoplastic resin foam by the method, electric servomotors ( 31, 32 ) are employed to drive the screw ( 20 ). After the step of plasticizing the resin material has been completed, brakes are applied to the electric servomotors ( 31, 32 ) to prevent the screw ( 20 ) from retreating. Alternatively, even after the step of plasticizing the resin material has been completed, the screw ( 20 ) is driven at low speeds in the direction of plasticization until immediately before the step of injecting the melted resin is initiated. This allows foaming inside the screw cylinder ( 1 ).

BACKGROUND OF INVENTION

1. Field of Invention

The present invention relates to an apparatus and method for forming athermoplastic resin foam. The apparatus comprises a screw cylinder and ascrew provided drivably in the directions of plasticization andinjection inside said screw cylinder. The method comprises the step ofinjecting an inert gas such as a carbon dioxide gas or a nitrogen gas,having at least in pressure a pressure equal to or greater than asupercritical pressure into said screw cylinder. The method alsocomprises the steps of permeating melted resin with the inert gas undera supercritical state inside the screw cylinder, and injecting themelted resin, permeated by the inert gas, into a mold to provide athermoplastic resin foam.

2. Related Art

There have been suggested many apparatuses and methods for molding athermoplastic resin foam, for example, in Japanese Unexamined PatentPublication Hei 8-258096 and No. Hei. 10-230528. The methods include thesteps of melting thermoplastic resin inside the cylinder of aninjection-molding machine, permeating the melted thermoplastic resinwith an inert gas such as carbon dioxide gas or nitrogen gas under asupercritical state, and injecting the melted resin, permeated by thegas, into the mold. The apparatus for forming a fine foam, disclosed inthe aforementioned Japanese Unexamined Patent Publication Hei. 8-258096,is constructed in general as follows. That is, the apparatus comprises aheating cylinder, a main screw provided inside the heating cylinder, amixing screw provided on the top end portion of the main screw, and aninert gas supply unit for supplying an inert gas to the mixing screwportion. Thus, when the main screw is rotatably driven to feedpelletized resin material toward the top end portion of the heatingcylinder, the pelletized resin material is melted and then furtheruniformly melted with the mixing screw. At this time, when a carbondioxide gas is supplied to the melted resin, the carbon dioxide gaspermeates the melted resin material. Injecting the melted resin materialpermeated by the carbon dioxide gas into a mold by driving the mainscrew in the axial direction will provide a fine foam. On the otherhand, the apparatus for forming a thermoplastic resin foam, disclosed inJapanese Unexamined Patent Publication Hei. 10-230528, is constructed asfollows. That is, the apparatus comprises two separate units such as asuccessive plasticizing unit having a heating cylinder and a screw, andan injecting unit having a plunger. Thus, the two units can also providea thermoplastic resin foam as follows. That is, when the screw isrotatably driven to melt the pelletized resin material and a carbondioxide gas is supplied to the melted resin, the carbon dioxide gaspermeates the melted resin material. The melted resin material permeatedby the carbon dioxide gas is injected into the mold of the injectingunit having the plunger by driving the screw in the axial direction.Then, the plunger is driven to provide a thermoplastic resin foam in thesame manner.

However, for example, the critical pressure of the carbon dioxide gas is7.4 MPa. There is an unsolved problem of sealing the carbon dioxide gasinside the heating cylinder, which will arise when the carbon dioxidegas under a supercritical state with a pressure considerably higher thanthe aforementioned pressure is injected into the melted resin materialinside the heating cylinder. Furthermore, the apparatus for forming athermoplastic resin foam is complicated in structure and comparativelyexpensive.

SUMMARY OF INVENTION

The present invention was developed to provide a method for forming athermoplastic resin foam, by which the aforementioned problems can besolved. More specifically, the object of the invention is to provide amethod and an apparatus for forming a thermoplastic resin foam, by whichfoaming can be prevented inside the screw cylinder to provide ahigh-quality fine foam.

To achieve the aforementioned object, the present invention provides anapparatus for forming a thermoplastic resin foam, which is constructedas follows. That is, the apparatus comprises a screw cylinder having amaterial supply hole arranged near a rear end portion of one end of thescrew cylinder, and an injection nozzle arranged on a top end portion ofthe other end of the screw cylinder. The apparatus also comprises ascrew provided in said screw cylinder for being rotatably driven indirections of plasticization and injection, and a drive means fordriving said screw in the directions of plasticization and injection.The apparatus is constructed such that said screw corresponds to saidscrew cylinder and is selected as a first metered, or plasticizing,portion, a low-pressure portion, and second metered, or plasticizing,portion in that order from the rear end portion to the top end portion.A gas supply hole for injecting an inert gas is disposed at a positioncorresponding to the low-pressure portion of said screw of said screwcylinder. The inert gas is a carbon dioxide gas or a nitrogen gashaving, at least in pressure, a pressure equal to or greater than asupercritical pressure or under a supercritical state. Said drive meanscomprise an electric servomotor.

The present invention provides a method for forming a thermoplasticresin foam comprising the step of plasticizing a thermoplastic resinmaterial by rotatably driving a screw provided drivably in directions ofplasticization and injection inside a screw cylinder. The method alsocomprises the step of injecting melted resin, permeated by an inert gas,into a mold by driving said screw in the direction of injection afterhaving injected the inert gas into said screw cylinder to allow theinert gas to permeate melted resin. Here, the inert gas is a carbondioxide or a nitrogen gas having, at least in pressure, a pressure equalto or greater than a supercritical pressure or under a supercriticalstate. The method employs an electric servomotor as a drive means fordriving said screw in the directions of plasticization and injection.After the step of plasticizing the thermoplastic resin material has beencompleted, said screw is also prevented from retreating by applyingbrake to said electric servomotor to thereby maintain a pressure at asupercritical pressure or more inside said screw cylinder.

The present invention provides a method for forming a thermoplasticresin foam comprising the step of plasticizing a thermoplastic resinmaterial by rotatably driving a screw provided drivably in directions ofplasticization and injection inside a screw cylinder. The method alsocomprises the step of injecting melted resin, permeated by an inert gas,into a mold by driving said screw in the direction of injection afterhaving injected the inert gas into said screw cylinder to allow theinert gas to permeate melted resin. Here, the inert gas is a carbondioxide or a nitrogen gas having, at least in pressure, a pressure equalto or greater than a supercritical pressure or under a supercriticalstate. The method employs an electric servomotor as a drive means fordriving said screw in the directions of plasticization and injection.Furthermore, the step of plasticizing the thermoplastic resin materialis performed until immediately before the step of injecting the meltedresin is initiated.

The present invention provides a method for forming a thermoplasticresin foam comprising the step of plasticizing a thermoplastic resinmaterial by rotatably driving a screw provided drivably in directions ofplasticization and injection inside a screw cylinder. The method alsocomprises the step of injecting melted resin, permeated by an inert gas,into a mold by driving said screw in the direction of injection afterhaving injected the inert gas into said screw cylinder to allow theinert gas to permeate melted resin. Here, the inert gas is a carbondioxide or a nitrogen gas having, at least in pressure, a pressure equalto or greater than a supercritical pressure or under a supercriticalstate. The method employs an electric servomotor as a drive means fordriving said screw in the directions of plasticization and injection.Furthermore, even after the step of plasticizing the thermoplastic resinmaterial has been completed, said screw is driven at low speeds in thedirection of plasticization until immediately before the step ofinjecting the melted resin is initiated.

The present invention provides a method for forming a thermoplasticresin foam comprising the step of plasticizing a thermoplastic resinmaterial by rotatably driving a screw provided drivably in directions ofplasticization and injection inside a screw cylinder. The method alsocomprises the step of injecting melted resin, permeated by an inert gas,into a mold by driving said screw in the direction of injection afterhaving injected the inert gas into said screw cylinder to allow theinert gas to permeate melted resin. Here, the inert gas is a carbondioxide or a nitrogen gas having, at least in pressure, a pressure equalto or greater than a supercritical pressure or under a supercriticalstate. The method employs an electric servomotor as a drive means fordriving said screw in the directions of plasticization and injection.Furthermore, when a pressure has dropped below a pre-set value insidesaid screw cylinder, said screw is driven in the direction ofplasticization so as to stop said screw when the pressure becomes equalto or greater than the pre-set value. This is carried out to maintainthe pressure at a supercritical pressure or more inside said screwcylinder.

The present invention provides a method for forming a thermoplasticresin foam comprising the step of plasticizing a thermoplastic resinmaterial by rotatably driving a screw provided drivably in directions ofplasticization and injection inside a screw cylinder. The method alsocomprises the step of injecting melted resin, permeated by an inert gas,into a mold by driving said screw in the direction of injection afterhaving injected the inert gas into said screw cylinder to allow theinert gas to permeate melted resin. Here, the inert gas is a carbondioxide or a nitrogen gas having, at least in pressure, a pressure equalto or greater than a supercritical pressure or under a supercriticalstate. The method employs an electric servomotor as a drive means fordriving said screw in the directions of plasticization and injection.Furthermore, when said screw is driven in the direction ofplasticization, driving said screw in the opposite direction ofplasticization is intermittently combined therewith.

The present invention provides a method for forming a thermoplasticresin foam comprising the step of plasticizing a thermoplastic resinmaterial by rotatably driving a screw provided drivably in directions ofplasticization and injection inside a screw cylinder. The method alsocomprises the step of injecting melted resin, permeated by an inert gas,into a mold by driving said screw in the direction of injection afterhaving injected the inert gas into said screw cylinder to allow theinert gas to permeate melted resin. Here, the inert gas is a carbondioxide or a nitrogen gas having, at least in pressure, a pressure equalto or greater than a supercritical pressure or under a supercriticalstate. The method employs an electric servomotor as a drive means fordriving said screw in the directions of plasticization and injection.Furthermore, during plasticization, said screw cylinder is provided withmicro-vibration in the direction of injection.

To achieve the aforementioned object, the present invention provides anapparatus for forming a thermoplastic resin foam, which is constructedas follows. That is, the apparatus comprises a screw cylinder having amaterial supply hole arranged near a rear end portion of one end of thescrew cylinder, and an injection nozzle arranged on a top end portion ofthe other end of the screw cylinder. The apparatus also comprises ascrew provided in said screw cylinder for being rotatably driven indirections of plasticization and injection, and a drive means fordriving said screw in the directions of plasticization and injection.The apparatus is constructed such that said screw corresponds to saidscrew cylinder and is selected as a first metered, or plasticizing,portion, a low-pressure portion, and second metered, or plasticizing,portion in that order from the rear end portion to the top end portion.A gas supply hole for injecting an inert gas is disposed at a positioncorresponding to the low-pressure portion of said screw of said screwcylinder. The inert gas is a carbon dioxide gas or a nitrogen gashaving, at least in pressure, a pressure equal to or greater than asupercritical pressure or under a supercritical state. Said drive meanscomprise an electric servomotor.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional schematic front view illustrating part of anapparatus for forming a thermoplastic resin foam according to anembodiment of the present invention.

FIG. 2 is an enlarged schematic front view illustrating part of a screwdrive unit of an apparatus for forming a thermoplastic resin foamaccording to an embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

First, an embodiment of an in-line apparatus for forming a thermoplasticresin foam is explained which is used for carrying out the presentinvention. FIG. 1 is a cross-sectional schematic front view illustratingpart of the embodiment of the apparatus for forming a thermoplasticresin foam. As shown in FIG. 1, the apparatus for forming athermoplastic resin foam according to the present embodiment comprisesgenerally a screw cylinder 1, a screw 20, and a screw drive unit 30.Here, the screw 20 is rotatably driven into the interior of the screwcylinder 1 in the direction of plasticization, also being rotatablydriven in the axial direction or the direction of injection. The screwdrive unit 30 is adapted to drive the screw 20 in the direction ofplasticization and in the direction of injection.

The screw cylinder 1 has a predetermined length in the axial directionand is provided with a gas supply hole 2. The gas supply hole 2 islocated approximately at the middle portion of the screw cylinder 1 toreach from the outside of the screw cylinder 1 to the inside thereof.Through the gas supply hole 2, an inert gas is supplied which has apressure equal to or greater than a supercritical gas pressure or asupercritical state. In addition, the gas supply hole 2 is hermeticallyconnected with a gas pipe 3, which is in communication with an inert gassupply unit. In this embodiment, for example, an inert gas such as acarbon dioxide gas or a nitrogen gas is injected into a melting resinmaterial at a pressure equal to or greater than a supercritical gaspressure of 7.4 MPa (for a carbon dioxide gas). For this reason, thereare provided a compressor, a pressure control valve, and the like insidethe unit for supplying an inert gas. Incidentally, the supercriticaltemperature of an inert gas is reached in the screw cylinder 1 whenplasticization is being carried out. Therefore, when an inert gas whichis at room temperature or has been preheated with waste heat is supplieddirectly to the screw cylinder 1, the supercritical temperature willalso be reached. Thus, there is provided no additional heating unit onthe inert gas supply unit. However, as a matter of course, such an inertgas can be supplied to the gas supply hole 2 that is under asupercritical state in pressure and temperature.

The screw drive unit 30 is provided at the rear end portion of the screwcylinder 1. As shown in FIG. 2 with an enlarged view, the screw driveunit 30 comprises an electric injection servomotor 31 and an electricplasticization servomotor 32. A ball screw 33 is fixed to the outputshaft of the electric injection servomotor 31, and a ball nut 34 isthreaded over the ball screw 33. This arrangement allows the ball nut 34to be driven in the axial direction when the electric injectionservomotor 31 rotatably drives the ball screw 33. Alternatively, drivingthe ball nut 34 in the axial direction will cause the ball screw 33 torotate. The ball nut 34 is connected to a drive body 36 via a thrustbearing 35. In addition, the screw 20 is connected to the top end of theoutput shaft 36′ of the drive body 36.

There is formed a spline on the outer circumference of the rotationalshaft 38 of the electric plasticization servomotor 32. A ball spline 39is attached by insertion to the rotational shaft 38 on which the splineis formed. This arrangement prevents the ball spline 39 from moving inthe rotational direction with respect to the rotational shaft 38 butallows it to freely move in the axial direction. The outer circumferenceof the ball spline 39 is toothed to mate with a large gear 37 that isfixed to the outer circumference of the drive body 36.

This embodiment is also provided with a controller 40 comprising acontrol device and a timer. The electric injection servomotor 31 is tobe controlled in a predetermined manner, as described later, by means ofthe controller 40 via an injection electric-servomotor control portion41. The electric plasticization servomotor 32 is also controlled in thesame manner via a plasticization electric-servomotor control portion 42.A setting device installed in the controller 40 is adapted to providethe following various values required for plasticization. For example,the values include the upper and lower limits of the pressure of inertgases, timing for supplying inert gases, timer settings for stopping thesupply, rotation speeds of the electric servomotors 31, 32 of the screwdrive unit 30, and back-pressures for plasticization. The values alsoinclude the temperature of the heaters 9, which are provided on theouter circumference of the screw cylinder 1 and an injection nozzle 5.The electric servomotors 31, 32 are to be controlled with the controller40, for example, by feedback control so as to maintain theaforementioned various types of values to the predetermined settings.

Referring to FIG. 1, there is shown a measurement chamber 4 to the leftof the screw cylinder 1 near the top end portion thereof, and theinjection nozzle 5 is provided at the top end portion. The injectionnozzle 5 is provided with a shut-off valve 6. On the outer circumferenceof the screw cylinder 1 and the injection nozzle 5, a plurality ofheaters 9 are provided, the temperature of each of which is controlled.Thus, for example, a temperature of 100° C. or more, which is equal toor greater than a supercritical temperature, is maintained inside thescrew cylinder 1. In addition, there is provided a first pressure gauge15 downstream of the gas supply hole 2 of the screw cylinder 1. There isalso provided is a second pressure gauge 16 in the measurement chamber4. The pressure measurements provided by these pressure gauges 15, 16are to be inputted to the controller 40. Incidentally, in FIG. 2, theelectric injection servomotor 31 is adapted to rotate slightly in theforward and reverse directions during plasticization to apply finevibration to the screw 20 in the direction of injection.

In this embodiment, a controlled amount of a thermoplastic resinmaterial J is to be supplied to the screw cylinder 1. FIG. 1 shows asupply cylinder 11 of a hopper 10 being directly connected to a materialsupply hole 7 of the screw cylinder 1. Controlling the rotational speedof the screw is to control the amount of the thermoplastic resinmaterial J that is supplied from the hopper 10 to the screw cylinder 1and thus control the amount of melting resin at a low pressure portionT, described later.

The screw 20 moves in the axial direction upon plasticization andinjection. As shown in FIG. 1, the rear end portion of the screw 20 is afirst stage S1 and the top end portion thereof is a second stage S2,corresponding to the screw cylinder 1 to some extent. The first stage S1comprises a supply portion K and a first metered (plasticizing) portionM1 after or downstream of the supply portion K. The supply portion Kcorresponds to the material supply hole 7 of the screw cylinder 1 andhas a comparatively deep screw groove 21. The screw groove 21 changesgradually in depth from the groove of the supply portion K to the grooveof the first metered (plasticizing) portion Ml, the screw groove 21 ofthe first metered portion Ml being shallower in depth. The thermoplasticresin material J fed from the supply portion K by the rotation of thescrew 20 melts, while being subjected to heat provided by the heaters 9installed on the screw cylinder 1, compression, and shearing. Thus, thethermoplastic resin material J melts completely at the first meteredportion Ml. This prevents leakage of an injected inert gas toward thesupply portion K. That is, the melted resin provides for sealing.

The second stage S2 comprises the low-pressure portion T located afterthe first stage S1 and a second metered (plasticizing) portion M2downstream thereof. The screw groove 21 of the low-pressure portion T isdeeper. This causes a reduction in pressure of the melted resin fed fromthe first stage S1 to keep the low-pressure portion T at a low pressureeven when filled with the melted resin. In some cases, a starved feedportion is produced. Consequently, it is made easy to inject an inertgas to the low-pressure portion T. In addition, the low-pressure portionT is adapted to have a length enough to cover the gas supply hole 2 whenthe screw 20 has moved in the axial direction. The screw groove 21 ofthe second metered portion M2 is shallow in depth, being filled with themelted resin. This allows the inert gas injected to be sealed with themelted resin of the second metered portion M2.

Incidentally, in the aforementioned embodiment, the screw groove 21 ofthe low-pressure portion T of the screw 20 becomes deeper in depth toprovide an increased volume between flights 22 and 22. It is alsopossible, however, to provide an increased volume between the flights 22and 22 by narrowing the width of the flights 22 instead of making thescrew groove 21 deeper. Moreover, it is also possible to provide anincreased volume between the flights 22 and 22 by widening the pitch ofthe flights 22, 22. Moreover, it is obvious that the screw groove 21 canbe made deeper in depth and the flights 22, 22 can be narrowed in widthto widen the pitch of the flights 22, 22.

Now, another example is explained below, in which the aforementionedapparatus for forming a thermoplastic resin foam is employed. First, thethermoplastic resin material J is put into the hopper 10. The completiontime of plasticization is set such as to the position of the screw 20with the setting device provided on the controller 40. It is alsoprogrammed to brake the electric plasticization servomotor 32 and theelectric injection servomotor 31 when the completion time ofplasticization is detected which has been set. In addition, variousvalues necessary for plasticization are set. These values include theupper and lower pressure limits of the inert gas, the pressure of thesecond metered portion M2, the pressure inside the measurement chamber4, the temperature of the heaters 9, and the rotational speed of thescrew 20. Then, the shut-off valve 6 is closed to drive the screw. Thethermoplastic resin material J is then supplied to the screw cylinder 1at the pre-set ratio. The electric plasticization servomotor 32 isactivated to allow the rotational shaft 38 to rotate. This causes thelarge gear 37 to be driven via the ball spline 39, and thus the screw 20is rotatably driven at a predetermined speed. While being fed by therotation of the screw 20, the thermoplastic resin material J suppliedfrom the hopper 10 melts by the heat applied from outside and producedby the shearing and frictional effects of the rotation of the screw 20,as conventionally known. Thus, the thermoplastic resin material J is fedto the first metered portion Ml, where it melts completely. Then, it isfed to the second stage S2. At this time, the temperature inside thescrew cylinder 1 has reached the supercritical temperature of the inertgas, for example, a temperature of 100□ C. or more. Accordingly, thethermoplastic resin material J is accumulated in the measurement chamber4 at the front of the screw cylinder 1. The screw 20 will retreat due tothe pressure of the resin in proportion to the amount of the resinaccumulated. The retreat of the screw 20 will cause the drive body 36and the ball nut 34 to retreat. The ball screw 33 will rotate againstthe pre-set torque of the electric injection servomotor 31. This therebyprovides a predetermined back pressure.

When the timer of the controller 40 has counted to the pre-set time uponmeasurement in the foregoing, an inert gas is injected from the gassupply hole 2 to the low-pressure portion T of the second stage S2. Theinert gas is a gas under the supercritical state such as a carbondioxide gas or a nitrogen gas, or a gas at a pressure equal to orgreater than the supercritical pressure. The melted resin in the firstmetered portion Ml prevents the leakage of the injected inert gas towardthe supply portion K. In addition, upon injection of the gas, the screwgroove 21 of the low-pressure portion T is made deeper and the pressureof the melted resin made lower. This makes it possible to inject theinert gas at a comparatively low pressure of about a few Mpa to 20 MPa,which is equal to or greater than the supercritical pressure. Theinjected inert gas is at a supercritical temperature, for example, atemperature equal to or greater than 100□ C. in the screw cylinder 1.This makes it easy for the inert gas to permeate the melted resin by therotation of the screw 20. Thus, the thermoplastic resin material J isfed to the second metered portion M2 metered portion M2 of the secondstage S2. During this step, the inert gas is supplied so as not toprevent the second metered portion M2 from being lowered below thesupercritical pressure. Here, the melted resin in the second meteredportion M2 prevents the forward leakage of the injected inert gas. Thetiming for supplying and stopping the inert gas can also be controlledwith the position of the resin being measured.

The melted resin, which the inert gas has permeated, is fed to themeasurement chamber 4. As measurement proceeds, the screw 20 retreatsbackward by the pressure of the resin measured. At this time, thepressure of the measurement chamber 4 is measured with the secondpressure gauge 16. A pressure is applied to the screw 20 in thedirection of injection for measurement so as not to allow the measuredpressure to go below the supercritical pressure. Thus, the back pressureis required to be the supercritical pressure or greater. When the screw20 has retreated a predetermined amount, this is detected and themeasurement is completed.

After the measurement has been completed, this is detected to apply thebrake to the electric injection servomotor 31. The brake is also appliedto the electric plasticization servomotor 32. This prohibits therotation of the ball screw 33 to prevent the screw 20 to retreat due tothe pressure of the melted resin. Consequently, the melted resin inwhich the inert gas has been dispersed is prevented from being reducedin pressure to prevent foaming inside the screw cylinder 1. Thus, themelted resin is allowed to foam inside a mold to provide a finethermoplastic resin foam. Incidentally, the aforementioned effect canalso be obtained by applying the brake to the members associated withthe screw 20 instead of the electric injection servomotor 31 to preventthe rearward movement of the screw 20. Therefore, application of braketo the electric injection servomotor 31 includes the application ofbrake to the members associated with the screw 20.

Now, the thermoplastic resin material J finds itself in the injectionstep. The electric injection servomotor 31 is activated to rotatablydrive the ball screw 33, then the ball nut 34 being driven in the axialdirection. This causes the drive body 36 to be driven in the axialdirection and the screw 20 to be driven in the direction of injection.The shut-off valve 6 is opened to inject the thermoplastic resinmaterial J to a mold (not shown). The inert gas is kept being suppliedduring the injection step. When the time has counted to the pre-settime, the injection is stopped. The mold is opened after the resininjected into the mold has cooled and solidified. Thus, a thermoplasticresin foam is provided which has an average cell diameter of 0.01 to 50μm and an average cell density of 10⁸ to 10¹⁶ pieces/cm³. Hereinafter,molding is repeated in the same manner. The inert gas can also beinjected only during plasticization.

This embodiment also allows molding to be carried out in a way otherthan the aforementioned one by setting molding conditions different fromthe aforementioned ones to the controller 40. For example, it is alsoallowed to set all the conditions in the same way except that theelectric plasticization servomotor 32 performs plasticizationimmediately before the injection step by means of the electric injectionservomotor 31. This allows the screw 20 to retreat providing no time forreduction in pressure inside the screw cylinder 1, thereby preventingfoaming inside the screw cylinder 1. It is also allowed to drive thescrew 20 at low speed in the direction of plasticization from the end ofthe plasticization step until the injection step is initiated. Thisobviously makes it possible to prevent a reduction in pressure insidethe screw cylinder 1.

Suppose that the pressure of the screw cylinder 1, detected with thefirst and second pressure gauges 15, 16, is inputted into the controller40 and the detected pressure has reduced to the pre-set lower limit. Inthis case, the electric plasticization servomotor 32 can also becontrolled to be activated to drive the screw 20 in the direction ofplasticization and then stopped when the pre-set pressure has beenreached. This also prevents the foaming inside the screw cylinder 1. Inaddition, although the electric plasticization servomotor 32 isfrequently driven in the forward direction along the direction ofplasticization in general, a small step of reverse rotation may beincorporated into the forward rotation. That is, the screw 20 can berotated in the reverse direction to produce dense and non-denseportions, facilitating permeation and dispersion of the inert gas in themelted resin.

As described above, according to the present invention, the screw isselected, corresponding to the screw cylinder, as the first plasticizingportion, the low-pressure portion, and the second plasticizing portion,from the rear to the front end portion. The gas supply hole is providedat the position corresponding to said low-pressure portion of the screwon the screw cylinder. This provides an effect of injecting easily intothe screw cylinder an inert gas having a pressure equal to or greaterthan the supercritical pressure or an inert gas under the supercriticalstate. The gas supply hole is located between the first and secondplasticizing portions. At this time, this provides accordingly such aneffect that the injected inert gas is sealed with the melted resin atthe first and second plasticizing portions. In addition, a drive meansfor driving the screw in the directions of plasticization and injectioncomprises electric servomotors. This allows for composite operation ofthe screw during molding, especially during the plasticization step.Thus, such an effect unique to the present invention is provided thatfoaming can be prevented inside the screw cylinder to provide ahigh-quality thermoplastic resin foam.

In addition, after the plasticization step, the following inventionsmake it possible to prevent a reduction in pressure of the melted resinin which the inert gas has been dispersed inside the screw cylinder.Here, the inventions include the invention of preventing the screw fromretreating by applying brake to the electric servomotor, and theinvention of performing the plasticization step immediately before theinjection step. The inventions also include the invention of driving thescrew at low speed in the direction of plasticization until immediatelybefore the injection step is initiated. Also included is the inventionof driving the screw in the direction of measurement when the pressurehas reduced below the pre-set value inside the screw cylinder or ofstopping the screw when the pre-set pressure has been reached. Thus,such an effect unique to the aforementioned inventions can be obtainedthat the foaming is prevented inside the screw cylinder to provide afine high-quality thermoplastic resin foam. Furthermore, upon drivingthe screw in the direction of plasticization, an effect of acceleratingthe permeation and making uniform dispersion of the inert gas in themelted resin can be obtained by the following invention. That is, theinvention of combining intermittently the driving of the screw in thedirection of plasticization with the driving of the screw in the reversedirection of plasticization, or rotating the screw cylinder in thebackward direction during plasticization provides the same effect. Onthe other hand, such an effect can be further obtained in addition tothe aforementioned effect of preventing the foaming inside the screwcylinder, and accelerating the permeation and producing uniformdispersion of the inert gas in the melted resin in the followinginvention. The additional effect provided by the following invention isthat the inert gas can be easily injected into the screw cylinder toseal the injected inert gas with the melted resin in the first andsecond metered portions. The invention is limited among theaforementioned inventions in that an inert gas, at least in pressure,equal to or greater than the supercritical pressure or under thesupercritical state is injected to the position corresponding to thelow-pressure portion of said screw of the screw cylinder. Here, saidscrew is employed which is selected as the first metered portion, thelow-pressure portion, and the second metered portion.

What is claimed is:
 1. A method for forming a thermoplastic resin foamcomprising the steps of: plasticizing a thermoplastic resin material byrotatably driving a screw provided drivably in a direction ofplasticization and injection inside a screw cylinder, injecting meltedresin, permeated by an inert gas, into a mold by driving said screw inthe direction of injection after having injected the inert gas such as acarbon dioxide or a nitrogen gas having, at least in pressure, apressure equal to or greater than a supercritical pressure or the inertgas under a supercritical state into said screw cylinder to allow theinert gas to permeate melted resin, wherein an electric servomotor isused as a drive means for driving said screw in the direction ofplasticization and injection, wherein the step of plasticizing thethermoplastic resin material is performed until immediately before thestep of injecting the melted resin is initiated, wherein a controller isprovided with a setting device which includes pressure at a pre-setvalue for the screw cylinder, and when a pressure has dropped below thepre-set value inside said screw cylinder, the controller causes theelectroservomotor to drive said screw in the direction of plasticizationand to stop said screw when the pressure becomes equal to or greaterthan the pre-set value in order to maintain the pressure at asupercritical pressure or more inside said screw cylinder.
 2. A methodfor forming a thermoplastic resin foam as claimed in claim 1, whereinafter the step of plasticizing the thermoplastic resin material has beencompleted, said screw is prevented from retreating by applying brake tosaid electric servomotor to maintain a pressure at a supercriticalpressure or more inside said screw cylinder.
 3. A method for forming athermoplastic resin foam according to claim 2, wherein said screwcorresponds to said screw cylinder and is selected as a first meteredportion, a low-pressure portion, and second metered portion, in orderfrom a rear end portion to a top end portion of said screw; and furthercomprising: an inert gas having, at least in pressure, a pressure equalto or greater than a supercritical pressure or an inert gas under asupercritical state is injected into a position corresponding to thelow-pressure portion of said screw of said screw cylinder.
 4. A methodfor forming a thermoplastic resin foam as claimed in claim 1, whereinduring plasticization, said screw cylinder is provided withmicro-vibration in the direction of injection.
 5. A method for forming athermoplastic resin foam according to claim 4, wherein said screwcorresponds to said screw cylinder and is selected as a first meteredportion, a low-pressure portion, and second metered portions inthat-order from a rear end portion to a top end portion of said screw;and further comprising: an inert gas having, at least in pressure, apressure equal to or greater than a supercritical pressure or an inertgas under a supercritical state is injected into a positioncorresponding to the low-pressure portion of said screw of said screwcylinder.
 6. A method for forming a thermoplastic resin foam accordingto claim 1, wherein said screw corresponds to said screw cylinder and isselected as a first metered portion, a low-pressure portion, and secondmetered portions in order from a rear end portion to a top end portionof said screw; and further comprising: an inert gas having, at least inpressure, a pressure equal to or greater than a supercritical pressureor an inert gas under a supercritical state is injected into a positioncorresponding to the low-pressure portion of said screw of said screwcylinder.
 7. A method for forming a thermoplastic resin foam comprisingthe steps of: plasticizing a thermoplastic resin material by rotatablydriving a screw provided drivably in a direction of plasticization andinjection inside a screw cylinder, injecting melted resin, permeated byan inert gas, into a mold by driving said screw in the direction ofinjection after having injected the inert gas such as a carbon dioxideor a nitrogen gas having, at least in pressure, a pressure equal to orgreater than a supercritical pressure or the inert gas under asupercritical state into said screw cylinder to allow the inert gas topermeate melted resin, wherein an electric servomotor is used as a drivemeans for driving said screw in the direction of plasticization andinjection, wherein even after the step of plasticizing the thermoplasticresin material has been completed, said screw is driven at low speeds inthe direction of plasticization until immediately before the step ofinjecting the melted resin is initiated.
 8. A method for forming athermoplastic resin foam according to claim 7, wherein said screwcorresponds to said screw cylinder and is selected as a first meteredportion, a low-pressure portion, and second metered portions, in thatorder from a rear end portion to a top end portion of said screw; andfurther comprising: an inert gas having, at least in pressure, apressure equal to or greater than a supercritical pressure or an inertgas under a supercritical state is injected into a positioncorresponding to the low-pressure portion of said screw of said screwcylinder.
 9. A method for forming a thermoplastic resin foam comprisingthe steps of: plasticizing a thermoplastic resin material by rotatablydriving a screw provided drivably in a direction of plasticization andinjection inside a screw cylinder, injecting melted resin, permeated byan inert gas, into a mold by driving said screw in the direction ofinjection after having injected the inert gas such as a carbon dioxideor a nitrogen gas having, at least in pressure, a pressure equal to orgreater than a supercritical pressure or the inert gas under asupercritical state into said screw cylinder to allow the inert gas topermeate melted resin wherein an electric servomotor is used as a drivemeans for driving said screw in the direction of plasticization andinjection, wherein the step of plasticizing the thermoplastic resinmaterial is performed until immediately before the step of injecting themelted resin is initiated, wherein when said screw is driven in thedirection of plasticization, driving said screw in the oppositedirection of plasticization is intermittently combined therewith.
 10. Amethod for forming a thermoplastic resin foam according to claim 9,wherein said screw corresponds to said screw cylinder and is selected asa first metered portion, a low-pressure portion, and second meteredportion, in order from a rear end portion to a top end portion of saidscrew; and further comprising: an inert gas having, at least inpressure, a pressure equal to or greater than a supercritical pressureor an inert gas under a supercritical state is injected into a positioncorresponding to the low-pressure portion of said screw of said screwcylinder.